Did I ever tell you about a non-fiction book called The Lost Cosmonaut? It's by/about a guy who in the late 1990s or thereabouts visited a number of lesser-known former Soviet satellite republics, basically just to see what, if anything, was there.

In the last section of the book he goes to some place where allegedly the locals are still pagans. I can't recall what ethnic group they actually belong to*, because this book is full of peoples who are completely unknown outside the area they live, except maybe for one reference in an ethnographic paper from the 1920s or something. Anyway, he watches a pagan ceremony and visits for a while with the high priest, and quickly figures out that (a) the guy is the high priest because no one else was interested in the job; and (b) he's made up most of the religion based on the few details he can recall hearing from his grandparents, plus borrowings from local legend and Russian Orthodox ritual to fill in the rest.

I remember when I first read this part I suddenly wondered how often this has had to be done in other cultures. I've no idea if the high priest's reboot took, or died out with him (I don't even know whether he's dead or alive at this point.)

* I looked it up -- they're the Mari.